Electronic ballasts typically include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are generally classified according to switching topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter provides a square wave output voltage. The square wave output voltage is processed by a resonant output circuit that provides high voltage for igniting the lamps and a magnitude-limited current for powering the lamps in a controlled manner.
Several types of existing electronic ballasts employ a driven inverter and a series resonant output circuit. In such ballasts, the inverter and output circuit must be protected from the potentially damaging voltages and currents that may result when a lamp fault condition occurs. Common lamp fault conditions include lamp removal, lamp failure (e.g. degassed lamp), and failure of the lamp to conduct current in a substantially normal manner (e.g., diode-mode lamp).
Many existing protection circuits are quite complex and require a large number of components. As a result, the ballast tends to be costly and difficult to manufacture. A number of protection circuits have functional limitations as well. For example, many do not provide automatic ignition of a replaced lamp. Further, for ballasts that power multiple lamps, a number of existing protection circuits do not accommodate "true-parallel" operation of the lamps. For example, removal or failure of a single lamp is often responded to by either shutting down the inverter or operating the inverter at an elevated frequency. In either case, the remaining "good" lamps are prevented from continuing to provide a normal level of useful illumination.
It is therefore apparent that a need exists for an electronic ballast that is protected against various lamp fault conditions, that provides automatic ignition of replaced lamps, and that accommodates true-parallel operation of multiple lamps, but that does not require extensive protection circuitry. Such a ballast would be highly cost-effective, manufacturable, and reliable, and would therefore represent a significant advance over the prior art.